1. Field of the Invention
The invention relates to high density digital magnetic storage systems of the type generally utilized for computer bulk storage. Magnetic disk storage systems utilizing non-contact recording and readback typify such systems.
2. Description of the Prior Art
Present day non-contact digital magnetic bulk storage systems utilize saturated-flux recording in combination with run-length limited codes for reducing cost per written information bit while enlarging the on-line capacity of the storage systems. Such codes, although alleviating the problems (inherent in recording of random data) of loss of timing clock and interference between flux transistions, operate at the cost of requiring the recording of a larger number of code bits than corresponding data bits. Multilevel recording (using greater than two levels) is not utilized to increase data storage density since the inherently non-linear characteristic of the magnetic interface does not reliably permit this recording format in general. Specifically, however, ternary recording would be possible in a non-linear channel by utilizing as the three states, positive saturation, negative saturation and zero flux. Since the zero flux level of ternary recording would not erase the previously recorded data, overwriting by new data would then require either a time consuming erase cycle or an additional erase transducer. For various reasons a separate erase transducer has been unfeasible in disk files. It is furthermore appreciated that communication theory indicates that contemporary commercial systems have not yet achieved bit densities that approach the theoretical limit propounded by Nyquist.
Independently of the above, partial response signalling has been developed for increasing the data rate capacity of communication channels such as telephone transmission systems. Partial response data transmission is disclosed in such U.S. Pat. Nos. as 3,388,330 issued June 11, 1968 entitled "Partial Response Multilevel Data System" and Pat. 3,492,578 issued Jan. 27, 1970 entitled "Multilevel Partial-Response Data Transmission". Although partial response has been considered for utilization in digital magnetic storage systems, heretofore this signalling technique has been contemplated as applying to storage systems utilizing saturated flux recording. It is believed that these systems considered in the prior art would be subject to timing recovery difficulties; and if multilevel recording were attempted, better automatic gain control than that of contemporary practice would be needed, and in any event the problem discussed above of old data erasure would be prevalent.
Additionally, in such prior art systems difficulties would be encountered in shaping the channel frequency spectral response to partial response signalling, particularly with respect to Class IV partial response which would be one of the more useful classes of partial response signalling in that it permits achieving the Nyquist rate with minimum channel bandwidth. Since partial response signalling requires linearly combining pulses in the data stream for precisely and accurately controlled intersymbol interference, the saturated non-linearity of the magnetic interface as aggravated at high data densities would tend to distort the partial response data combinations rendering meaningful data recovery difficult if not impossible.
Thus it is appreciated from the foregoing that the prior art applications of partial response signalling to magnetic medium storage systems, as far as the binary data to be stored is concerned, are limited to a binary data rate equal to the Nyquist pulse rate for minimum bandwidth saturated flux recording systems. It is appreciated that if, as discussed above, ternary amplitude pulses are recorded utilizing positive saturation, zero flux and negative saturation to represent the ternary levels and thereby achieving 50 percent increase in binary data rate utilizing the bandwidth afforded by the magnetic interface, a separate erase cycle or separate erase tranducer would be required to erase the old data. Either of these erase techniques is very unattractive in commercial disk storage systems.